CurtPalme.com Home Theater Forum

**kal**

I'm helping Moome on some design stuff. He will be adding a 'contrast' pot on some future boxes to allow the peak output signal voltage to be adjusted. Question is, what's the best adjustment range?

Marquee's like a "hot" input signal around 1V pp right?
Most sources are around 0.7V pp right?

So would a range from 0.7 to 1.0 best the best? Or between 0.6 and 0.8V? Something else?

Kal

papalek · Joined: 08 Mar 2006 Posts: 1536 Location: Longs SC

From my experience.

Barco likes it at about .7 And over 1.0 will take out the input board.

Ampro likes at least 1.0 My 4600 is close to 1.3 from an Extron box and loves it.

**AnalogRocks** · Posted: Wed Feb 20, 2008 4:48 pm Post subject:

**kal** · Posted: Wed Feb 20, 2008 5:12 pm Post subject:

Ampro's like 1.3 volts? Really? That high?

What concerns me is the statement that over 1V will take out a Barco input board. Is this all Barcos? Moome can't give users a feature to go over 1V if there's a chance that it'll fry a certain brand of projector!

Kal

**AnalogRocks** · Posted: Wed Feb 20, 2008 5:20 pm Post subject:

papalek · Joined: 08 Mar 2006 Posts: 1536 Location: Longs SC

The "HOT" signal to the Ampro allows me to run contrast at 70 on a 10 foot wide screen and have a very vibrant image. With out the Extron box I have to crank the contrast to the max and the image is dull looking.

I have unhooked the cables from my 4600 and hooked them up to a Barco 701 and in about a minute the Barco shut down and I had to replace the input board. At that time I did not realize the "HOT" signal I was feeding the Ampro was the problem and connected it back again with a second board. Same thing,wiped the board out in about a minute. Connected it with another board,to the cable before the Extron box and worked fine.

**AnalogRocks** · Posted: Wed Feb 20, 2008 5:59 pm Post subject:

Person99 · Joined: 09 Mar 2006 Posts: 4899 Location: Flower Mound, TX

**kal**

Nashou66 · Joined: 12 Jan 2007 Posts: 16171 Location: West Seneca NY

I think up to 1v is a good choice. the thing is is some people have been modding input boards with newer current feed back op amps and if they didnt bother to change out the feedback resistors to give the proper gain then then it might be too high. Most video board designs from what i hear and have read in my searches is that they all try to do a gain of +2. so that would be 2.7, I think, I never understood that measurement... is it that it doubles the input or adds +2? I always thought it was times 2 so .7*2=1.4. well back to what i was saying about people modding some chips could have a gain much higer if they dont changeout the feedback resistors afew of those op Amps have a gain of +5. That could be way to hot for the op amp on the neck board, or some of the switching multipliers and other chips on the vim.

I could be wrong in this theory but just a thought...

Athanasios

**Curt Palme** · Posted: Wed Feb 20, 2008 6:12 pm Post subject:

Nashou66 · Joined: 12 Jan 2007 Posts: 16171 Location: West Seneca NY

WTS · Joined: 08 Mar 2006 Posts: 1276 Location: Calgary

I was going to say, it's obvious most people don't have a clue what they are talking about when it comes to electronics, video in this case, but Curt beat me to it.

In the Barco and Marquee there are ICs that if over driven you'll get white crushing but it won't blow your input. Unless of course you feed it 110vac!

Nashou66 · Joined: 12 Jan 2007 Posts: 16171 Location: West Seneca NY

Hello Walter, glad to see you. I guess I am one of those people! Wink

But I'm trying to learn . Thanks with your help and advice you gave me. things are looking good.

Athanasios

**kal** · Posted: Wed Feb 20, 2008 8:36 pm Post subject:

9kids · Joined: 25 Jun 2007 Posts: 138 Location: Manassas, VA

I agree that you don't want to go to high for most projectors, but if there are a few models that can benefit from something over 1.0V, then what about a optional jumper that could be added or removed to increase the range to say 1.2 or 1.3V. Then a major disclaimer in the manual about not changing the jumper, because it could cause damage to your projector. You know, one of those undocumented features, that people who know what they are doing can play with. Don't we all love to find those undocumented features in our consumer electronics?

Person99 · Joined: 09 Mar 2006 Posts: 4899 Location: Flower Mound, TX

WTS · Joined: 08 Mar 2006 Posts: 1276 Location: Calgary

Well before anyone gets to excited about Moome doing something like that I'll have to ask him how he plans on implementing it into the circuit. Depending on how it's done you might be better off without it from a performance standpoint.

tse · Joined: 03 May 2006 Posts: 1014 Location: Sweatbucket, Fl.

My $0.02 worth. Video should always be 0.7Vpp (1Vpp if with sync) when properly terminated. That means 1.4Vpp unterminated.

Higher level signals can cause white crush, as mentioned earlier. Or not. You'll have to check with a scope or the proper test patterns to tell.

Higher level signals require the video input circuits to have a higher slew rate to maintain bandwidth. The Marquee VIM actually attenuates the input to reduce the slew rate needed.

There should only be one control per function in a system. That would be the contrast control. Turn it up to 100 if you have to. It's only a number.

Scott

Nashou66 · Joined: 12 Jan 2007 Posts: 16171 Location: West Seneca NY

Scott i have a question...what if you put op amps with higher slew rates than the original CLC449's? will they be able to handle the higher voltage or is there more involved?

Athanasios